Field of the Invention
The present invention relates to rectennas, and more particularly, to a rectenna for rectifying and converting radio waves into electrical power.
Description of the Related Art
Rectenna is a device that converts microwaves received by its antenna into direct current to obtain electrical power. Such rectennas are expected to be used for wireless power transmission, as in space solar power generation systems in which electricity is generated with the use of solar light collected by solar panels mounted on an artificial satellite and is transmitted to the ground in the form of microwaves, which are received on the ground and converted into electrical power, as well as in wireless power supplies for feeding electrical power to electric vehicles, mobile terminals and the like.
In recent years, rectennas have come to be used in RFID (Radio Frequency Identification) chips for the purpose of rectification. An RFID chip with a rectenna requires no battery cell to be mounted thereon and can obtain electrical power necessary for its operation by rectifying part of the carrier wave received by the antenna. Exemplary operation of such an RFID chip is schematically illustrated in FIG. 4. A carrier wave received by an antenna 102 passes through a transmission line 104 to a terminal to which the switching element of a switch 106 is connected. The switch 106 has three connection terminals, that is, a terminal 106a which opens the circuit when the switching element is connected thereto, a terminal 106b connected to a resistor 108, and a terminal 106c which short-circuits the circuit when the switching element is connected thereto.
The RFID chip configured as described above is capable of transmitting information written in a memory thereof as the position of the switch 106 is changed from one to another. When the switch 106 is connected to the terminal 106a so that the transmission line 104 may be open at its end opposite the antenna 102 as shown in FIG. 5A, impedance mismatch is caused, with the result that the carrier wave is totally reflected back in phase with the carrier wave input to the transmission line 104. On the other hand, when the switch 106 is connected to the terminal 106b, as shown in FIG. 5B, so that the transmission line 104 may be terminated by the resistor 108 with an impedance equal to that of the transmission line 104, the carrier wave is not reflected at all. Further, when the switch 106 is connected to the terminal 106c to short-circuit the transmission line 104 as shown in FIG. 5C, impedance mismatch is caused and the carrier wave is totally reflected back such that the phase thereof is opposite to that of the carrier wave input to the transmission line 104. Thus, by selectively opening and short-circuiting the transmission line 104 at its end by the switch 106 and thereby modulating the phase of the signal, it is possible to transmit information stored in the RFID chip from the antenna 102 (Hideyuki NEBIYA, Kotomi UETAKE, and Masahide HAYAMA, “Passive Backscatter RFID System”, Seminar at Tohoku Branch, the Institute of Electronics, Information and Communication Engineers, June 2002, pp. 1-3). With the configuration described above, the RFID chip requires no power supply, and also since a transmitter for outbound transmission of the information stored in the memory of the RFID chip is unnecessary, it is possible to realize a small-sized, batteryless RFID chip.
However, the technique disclosed in the above literature requires a physical space that allows the switch 106 to perform switching operation, and thus there is a limit on further reduction in size. Also, since the switch 106 is mechanically driven, deterioration in the switch 106 and like factors impose restrictions on durability.